In beryllium copper alloys, the resultant combination of properties of strength, ductility and conductivity are controlled by the amount, size and distribution of the precipitates and residual stresses. Therefore, the sequence and degree of work hardening and thermal aging which determine the kinetics of precipitation greatly influence the ultimate properties.
Various methods are known for improving the mechanical properties and for decreasing the distortion which results during the precipitation hardening of formed parts produced from beryllium copper alloys. Unfortunately, these known methods are only partially effective and often fail to reproducibly improve the strength, fatigue life and high temperature properties of aged parts to a commercially acceptable degree.
Basically, all presently available methods for producing precipitation hardened formed parts from beryllium copper alloys include the general combination of the following sequence of processing events: preparing a beryllium copper melt: casting the melt: hot working the cast alloy; solution annealing the alloy; cold working the solution annealed alloy with optionally further mill hardening: forming parts form the cold worked alloy; and aging the cold worked beryllium copper. Various modifications have been developed in an attempt to minimize the variations in the non-reproducible dimensional and mechanical property changes experienced in this processing sequence. The product of the general combination above or any of the modifications thereto will be referred to herein as a processed beryllium copper alloy.
In this connection, reference is made to the methods disclosed in U.S. Pat. No. 4,792,365 (Matsui), U.S. Pat. No. 4,705,095 (Gasper), U.S. Pat. No. 4,692,192 (Ikushima), U.S. Pat. No. 4,657,601 (Guha), U.S. Pat. No. 4,594,116 (Inagaki), U.S. Pat. No. 4,579,603 (Woodard), U.S. Pat. No. 4,541,875 (Woodard), U.S. Pat. No. 4,533,412 (Rotem), U.S. Pat. No. 4,425,168 (Goldstein), U.S. Pat. No. 4,394,185 (McClelland), U.S. Pat. No. 4,221,257 (Narasimhan), U.S. Pat. No. 3,658,601 (Britton), U.S. Pat. No. 3,138,493 (Smith), the article entitled "Local Atomic Arrangements in Age-Hardenable Alloys" authored by J. B. Cohen, Scripta Metallurgica. Vol. 22, 1988. page 933, the article entitled "Small Angle Scattering Experiments from A1-4 wt % Cu Single Crystals Containing G.P.I. Zones", authored by V. Gerold and E. Bubeck, Scripta Metallurgica. Vol. 22, 1988, page 953, the article entitled "Stress Relaxation in Bending of Copper Beryllium Alloy Strip", authored by A. Fox in Journal of Testing and Evaluation. Vol. 8, No. 3, May 1980, the article entitled "High Resolution Electron Microscope Observations on G.P. Zones in an Aged Cu-1.97 wt % Be Crystal", authored by V. A. Phillips and L. E. Tanner in Acta Metallurgica, Vol. 21, April 1973. The methods disclosed in these prior art sources are only partially successful in reproducibly improving the mechanical properties and minimizing the distortions in precipitation hardened finished products.
Gerold and Bubeck studied the nucleation of G.P. zones in a single crystal containing A1-4 wt % Cu. When the oriented single crystal had been compressed during the first part of its aging, they found a difference in the nucleation rates of the G.P. zones created under tension as compared to compression. This agrees with the results obtained on BeCu alloys and reported by Woodard and Ebert in U.S. Pat. No. 4,579,603. There, they presented evidence which indicated that the rate of formation of these G.P. zones and or precipitates is different, when they are formed under compressive residual stresses, from the rate at which they are formed under tensile residual stresses.
The new model for the aging of Al-Cu alloys as proposed by Gerold and Bubeck suggests a coarsening of the Cu-rich platelets so that the thick zones are essentially all copper atoms. Cohen reports that this model would lead to the expectation of similar coarsening of the G.P. zones in a Cu-10.9 atomic percent Be alloy during the aging sequence. He describes test results in which the G.P. zones have been found to consist of from one to eight layers containing Be and Cu atoms. In each layer, the percentage of alloying Be atoms varies from 84.6% to 98.1%. It is interesting to note that he lists other shapes that contain 100% Be atoms. Then one can deduce that the hardening mechanisms for the Al-Cu alloys and the Cu-Be alloys are similar.
In U.S. Pat. No. 4,579,603, Woodard and Ebert recognize the existence of precipitate patterns that are created during the post hot forming, cold forming and precipitation hardening operations by differences in the rates of their formation. If there is a precipitate pattern, then when the precipitates are given the normal short time anneal, there Will be left a residual pattern of reduced residual stresses as well as concentrates of beryllium atoms and undissolved precipitates. This residual pattern could form the basis for the memory, which becomes evident on aging, that the formed part has of its thermal and mechanical history.
U.S. Pat. No. 4,579,603 shows that the effects of this memory can be minimized by the formation of an opposite pattern created by the second reduction and annealing taught therein. Rotem, Guha as well as U.S. patent application Ser. No. 07/164,481 (Woodard and Ebert) use high temperature anneals which have the effect of reducing the residual stress patterns and precipitate particle sizes which contribute to the effects of this memory.
Narasimhan describes a continuous casting technique, for producing amorphous metal strip which does not involve a hot rolling step. When used with the proper heat transfer media, this technique, following one or more cold rolling steps, can give narrow BeCu alloy strip which has a minimum of precipitate patterns. The production costs for producing such strip would be appreciably less than for narrow strip produced by present commercially employed techniques.
It has been found by Britton and others that a decrease in the magnitude of the existing residual stresses, that we now know are created by immersing the specimen in a salt medium, results in a decrease in the magnitude of the shrinkage that occurs on aging. Smith has shown that immersing cold worked beryllium copper strip in a hot salt medium, while it is under tension increased its elongation. Fox has shown that in beryllium copper, stress relaxation and precipitation hardening can occur simultaneously at the same temperature. The Goldstein and the McClelland patents comprehend the importance of relieving residual stresses prior to the forming operation by the incorporation of a pre-aging technique. Based on the teaching of Fox, it can be understood that in thermal treatments, such as the Goldstein and McClelland pre-aging techniques, the two reactions occur simultaneously.
Thermal treatments such as pre-aging reduce the magnitude of the existing cold working and residual stress patterns that effect the precipitation hardening. On the other hand, these thermal treatments also promote the nucleation and growth of the precipitates formed during precipitation hardening.
However, all of the prior art methods yield inconsistent non-reproducible results. The prior art methods fail to realize that there are macro residual stresses created by the types of cold worked reductions and micro residual stresses, similar to the kinds present in composites, that are created by metal flow around inert and non-coherent particles such as the cobalt beryllides.
In U.S. Pat. No. 4,579,603 issued to Woodard and Ebert, the effects of differences in heating rates on the elongation and proportional limits created in BeCu wire were described. This wire had been given a severe reduction, annealed, given a slight reduction and then had been given further cold working before being subjected to heat treatment in two different media. In one case, segments of wire were up-quenched into a molten salt bath and held for different lengths of time before being down-quenched in oil to ambient temperature. In the other case, segments of wire were inserted into an inert gaseous medium, held at temperature for the same lengths of time before being down-quenched in oil. The resultant differences in the elongation and proportional limits created by these two techniques are shown in FIGS. (1) and (2).
These beryllium copper alloys are used in those applications which need their unusual spring characteristics as well as in electrical connectors where reproducible dimensional, mechanical and electrical properties in the finished product are important if zero defects are to be obtained by the end user. These, improvements are urgently needed in the industry.